The range of a radio communication link, for example the radio link used to connect a hand-held cellular telephone to a cellular base station, depends on a number of factors. Among these are receiver sensitivity at the base station, antenna gain at the receiver and transmitter, path loss, and RF transmitter power at the hand-held terminal. In general, a more powerful hand-held terminal will have greater capability to provide service within shielding structures such as buildings and to sustain conversations deeper into fringe areas.
Nevertheless, the advantages of having a powerful terminal are counterbalanced by a number of practical considerations that limit the amount of power usefully disposed to a hand-held terminal. Primary among these considerations are the need to conserve the draw of electrical energy from the terminal's small rechargeable battery, and the need to throttle power to the lowest useable level so as to avoid interference with other radio links active at the same time.
Under certain conditions, however, the need to limit power is relaxed, for example or when the cellular terminal is used in a home docking station to provide data, voice, or security-system-backup communications, or when the cellular terminal is operated with a hands-free cradle in a motor vehicle. In these and similar applications, the docking station or cradle makes external power available, often by way of an integral battery charger, thereby easing the battery-draw constraint. In both cases, however, the need remains to throttle power as appropriate to avoid unnecessary interference.
In order to minimize interference, a cellular base station measures the strength of the incoming signal it receives from the mobile unit, and instructs the mobile unit to adjust its radiated power to the lowest level adequate to sustain good quality communications. This enables the mobile station to maintain communications as it moves about and experiences variations in path loss. The power adjustments ordered by the base station must, of course, fall within the capability of the terminal to supply RF power, and must further fall within the practical constraints of energy draw.
Within the accepted industry standards for cellular telephones, terminal power is generally categorized by class. For example, a terminal designed for operation with a GSM system falls into one of five classes, where those classes have maximum peak power-output limits ranging from 20 Watts (Class 1) to 0.8 Watts (Class 5), and where the power adjustments commanded by the base station take place over 15 steps of 2 decibels (dB) each. Within the AMPS system used in North America and elsewhere, three classes are used which have a maximum transmit power of 6 dbW (Class 1), 2 dbW (Class 2), and -2 dbW (Class 3). Because of battery limitations, handheld terminals ordinarily operate as Class-3 devices. This unfavorably limits their capability to provide good quality service within buildings and in deep-fringe areas. To overcome such limitations when hand-held terminals are used with hands-free accessory cradles in motor vehicles, the prior art teaches three solutions to maximizing power generally within the aforementioned constraints on energy draw and interference. These solutions are called here the external booster, the high-powered terminal, and the power-exception terminal.
The external booster provides a second (external to the terminal) RF power amplifier and associated control circuitry as part of the hands-free cradle. When the hand-held terminal is captured by the cradle, its presence activates the second power amplifier (PA), which is inserted between the hand-held terminal and an external antenna. In practice, the power output level of the hand-held terminal itself is sometimes reduced upon insertion into the cradle so as not to overdrive the second PA. Together, the handheld terminal and the external booster work as a Class-I device within the cellular system. The second PA boosts the maximum RF power output typically to 3.0 Watts, thereby providing about 7 dB advantage over a 0.6 Watt hand-held terminal in deep fringe operation. In order to provide this advantage, however, the mobile unit incurs considerable expense, bulk, complexity, and duplication of resources. For example, U.S. Pat. No. 5,457,814, "Power boost system for cellular telephone," describes an external booster comprising two duplexers, an RF power amplifier, a fault detector, automatic-power control circuitry, and pulse-width modulation circuitry that provides a way of communicating control information between the booster and the hand-held terminal.
The second solution proposed by the prior art, called here the high-powered terminal, is taught in U.S. Pat. No. 4,636,741, "Multi-level power amplifying circuitry for portable radio transceivers," wherein a hand-held unit with an internal high-power PA senses its own insertion into a vehicle cradle, increases its status from Class-3 to Class-1 (3 Watts in the preferred embodiment), and re-registers itself with the cellular system under Station Class Mark 1. However, this solution has several drawbacks, including the need for a higher DC supply voltage to sustain the high demands of the power amplifier (PA) when operating in Class-I service, the need for different construction to accommodate the thermal demands of Class-I operation not normally encountered in Class-3 operation, and the added complexity needed to change the Station Class Mark between Class-3 and Class-1. For these reasons, a later U.S. Pat. No. 5,457,814, teaches against the high-powered terminal of the type disclosed in 4,636,741, noting that the approach leads to "increased weight, bulk and cost, and greatly diminishes the amplifier reliability of the radiotelephone."
A third solution, called here the power-exception terminal, is disclosed in U.S. patent application Ser. No. 08/728,681 filed Oct. 9, 1996 titled "RF Gain Enhancement for Cellular Telephone." This application describes a cooperative relationship between a vehicle's cradle and a Class-3 hand-held terminal. Upon insertion into the cradle, the terminal increases its power to the maximum level it can sustain within Class-3 operation, and disables its power-control apparatus. Although this solution has the great virtue of simplicity, it provides no increase in transmitter power over that which the Class-3 terminal has inherently within its grasp, and it incurs the risk, although perhaps slight, of creating undue interference with other radio links whenever its power-control apparatus is disabled.
In view of the limitations of the prior art as described above, there remains a need for a simple, inexpensive hand-held terminal that provides increased transmitter power to give better service indoors or in fringe areas, wherein the increased transmitter power is enabled when the terminal is inserted into a docking station used to provide an RF link in support of a portable computer, or into a vehicle's hands-free cradle, and yet wherein the terminal operates within known constraints imposed by battery capacity used as a hand-held device.